MDL-33144 display filetype icon and mimetype based on extension

- use finfo to determine mimetype of a file only when it is unknown from file extension
- display filetype icon and mimetype in filemanager based on file extension
- fixed a small issue with specifying extension instead of group in file_get_typegroup()
- allow only web-compartible image types when embedding into editor or using in database field
- created new groups web_video and web_audio to use in resource module

lib/form/editor.php

@@ -302,7 +302,7 @@ class MoodleQuickForm_editor extends HTML_QuickForm_element {
 
             $args = new stdClass();
             // need these three to filter repositories list
-            $args->accepted_types = array('image');
+            $args->accepted_types = array('web_image');
             $args->return_types = (FILE_INTERNAL | FILE_EXTERNAL | FILE_REFERENCE);
             $args->context = $ctx;
             $args->env = 'filepicker';